This freezing is known as "Fermi freeze". The easiest solution is to simply update to Mac OS X Lion. However, if you wish to remain on Mac OS X Snow Leopard, then you can follow this guide to set up an (essentially) permanent workaround to the issue.
Hackintoshes using Fermi (400 series) graphics cards freeze whenever the cards begin to idle. The solution? Play a video in the background indefinitely! It's not as bad as it sounds. In fact, with a simple AppleScript, you'll be able to forget that the Fermi freeze even exists.

Open AppleScript editor (you can search for it in Spotlight search, the little magnifying glass on the upper right of your Mac). AppleScript is a basic programming language by Apple for performing simple tasks. We're going to write a workaround to Fermi freeze with Quicktime player. Copy this into the AppleScript editor:

Basically, this little AppleScript opens up Quicktime and loops a video called "Boot.mov" indefinitely. The best part? You won't even be able to see it (hence the "set visible" line in the script). You can change Boot.mov into any video that you want. For example, if you wanted to loop a video called "Pie.mp4" in your System folder, and your hard drive was named "Macintosh HD", you would replace "Hard Drive:Boot.mov" with "Macintosh HD:System:Pie.mp4". Any video that you choose to loop should be small and soundless. Below is the download link for the video that I use (it's just a video of a blank white screen):